Twenty children’s handprints pop in bright colors against the black background of a billboard along the Massachusetts Turnpike. They look like elementary school art projects, but they are not: They represent the 20 first-graders shot and killed last week in Newtown, Conn.

The prints were painted on the billboard in Boston on Saturday by the nonprofit Stop Handgun Violence as a call for a federal ban on assault weapons.

“I want commuters to realize that those babies have died in vain unless they pick up the phone and force their spineless members of Congress in this country to do something,” said John Rosenthal, founder and president of Stop Handgun Violence, and owner of the billboard.

Last Friday, a 20-year-old Connecticut man forced his way into Sandy Hook Elementary School in Newtown and gunned down 20 students and six staff members with a Bushmaster AR-15-style semiautomatic rifle and two other firearms before committing suicide, according to authorities.

The massacre has sparked debate over the need for stricter gun laws, with many officials in the state, including Massachusetts Governor Deval Patrick and Boston Mayor Thomas M. Menino, calling for a federal ban on automatic weapons.

Massachusetts is one of a small number of states that continued a ban on assault weapons after Congress let a similar ban expire in 2004.

The hands were added to the billboard’s existing design, which features a single 20-foot-tall red hand print and a ticker that Rosenthal says counts the number of children who have died from gun violence since the 2010 election. On Saturday, that number was 6,248.

‘Adding these symbols of the unimaginable loss at Sandy Hook is a call to action for the motorists and passengers.’

In a statement, Menino thanked Rosenthal for adding the tiny hand prints to the billboard.

“Adding these symbols of the unimaginable loss at Sandy Hook is a call to action for the motorists and passengers who are all moved and horrified by this tragedy. Let’s all demand a plan to end gun violence,” said Menino in the statement. “There is so much work to do, I won’t rest until it is done, and I know the American public won’t either.”

The billboard stretches for 252 feet along the Mass. Pike near Fenway Park, said Rosenthal.

On Saturday afternoon, it caught the attention of drivers and pedestrians.

One man stopped to take pictures, and many paused to stare despite cold, harsh winds.

“It was good they added something, because it was such a huge, huge tragedy,” said Alexandria O’Neil, 24, who was out walking with her mother.

“Anything that will make people stop and think is good.” said her mother, Christine Bryant, 58.

Lew Sovocool, 31, who was visiting Boston from Durango, Colo., on his way to Maine for the holiday, said he is not against guns, but he doesn’t believe people need to have assault weapons.

“I feel pretty strongly about the Second Amendment, but I don’t think the founding ­fathers were thinking about rapid-fire assault weapons when they were writing the Bill of Rights,” he said.

Some said they were disappointed by the alteration of the billboard.

“I think they are seizing on this tragedy to pursue a political agenda that has nothing to do with what happened,” said Jesse Cohen, a Framingham lawyer who specializes in firearms law and consults for gun rights groups such as the Gun Owners’ Action League. “What we need to do is fix the security of our schools.”

Cohen said that both Stop Handgun Violence and the National Rifle Association go “too far,” and overreact to tragedies like the Newtown shooting in predictably dogmatic ways.

“All of us in the middle suffer from their battles back and forth,” he said. “That’s why the silent majority needs to speak up about sensible laws.”

Rosenthal dismissed that criticism, saying that gun violence is already a political issue, because gun manufacturers are protected by Congress.

“Toy guns and teddy bears are regulated,” he said. “But the real gun, that results in 3,000 children killed a year — no regulations.”

And if the hand prints seem shocking, he said, they pale in comparison with the stark images of the aftermath of the tragedy in Newtown.